Many products that we think of as strictly multiplayer also have single player modes. In many cases this single player mode helped adoption in the early stages when the network effects were not yet strong. For example, you could use Flickr just to store photos privately if you wanted to. I thought of Foursquare as strictly multiplayer until my Hunch cofounder Tom Pinckney told me he uses it solely to keep track of restaurants he’s gone to so he’ll remember which ones to go back to.